In production of steel, an SN device is used as a means to control a flow rate of molten steel to be discharged from a molten metal vessel, such as a ladle or a tundish. The SN device comprises two or three SN plates each made of a refractory material and formed with a nozzle hole. The SN plates are assembled such that they are slidably moved with respect to each other under a constrained condition while being superimposed on each other under a surface pressure, so as to adjust an opening between the nozzle holes to control a flow rate of molten steel.
Therefore, an SN plate is required to have spalling resistance against thermal stress during casting, as well as corrosion resistance against slag, inclusions, etc., in molten steel.
Heretofore, as means to improve corrosion resistance of an SN plate, it has been known to add a metal, such as aluminum or silicon (see, for example, Patent Document 1).